Half-toning method and apparatus for solid area coverage



N 1970 G. K. STARKWEATHER 3,540,806

HALF-TONING METHOD AND APPARATUS FOR SOLID AREA COVERAGE '2 Sheets-Sheet 1 Filed March 6, 1968 IIIIIII'IIIIIIIIIIIIIIIII INVENTOR. GARY K. STARKWEATHER N V- 1970 s. K. STARKWEATHER 3,540,805

HALF-TUNING METHOD AND APPARATUS FOR SOLID AREA COVERAGE Filed March 6, 1968 2 Sheets-Sheet 2 INVENTOR. GARY K. STARKWE ATHER rromve'rs 3,540,806 HALF-TONING METHOD AND APPARATUS FOR SOLID AREA COVERAGE Gary K. Starkweather, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Mar. 6, 1968, Ser. No. 710,938 Int. Cl. G03b 15/00 U.S. Cl. 355-3 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to xerography and in particular to apparatus for permitting full development of large solid areas and continuous-tone images placed on xerographic photoreceptors.

In xerography, a special xerographic photoreceptor comprising a layer of a photoconductive insulating material placed upon a conductive backing is used to support xerographic images. The photoreceptor may be formed in any shape. A latent image is formed by uniformly electrostatically charging the photoreceptive surface and then exposing it to a radiation pattern in the form of the image to be reproduced. This radiation selectively discharges areas of the photoreceptor forming an electrostatic charge pattern conforming to the radiation image. This radiation image is formed generally from an original document or other object which is illuminated and optically imaged on the photoreceptor.

The latent image on the photoconductive layer is then developed by contacting it with a finely divided electrostatically attractable material such as a resinous powder hereinafter called a toner. The toner is held to the image areas by electrostatic charge fields on the layer. The toner is held proportionately to the charge field so that the greatest amount of material is deposited where the greatest charge field is located. Where there is a minimum charge, there is little or no material deposited. Therefore, a toner image is produced to conform with the latent image previously placed on the photoreceptor. The toner may then be transferred to a sheet of paper or other surface and suitably fixed thereto to form a permanent print. This fixing may take place by heat or vapor which may fuse the toner to the support material to which it has been transferred.

The toner used to develop the image is generally formed in a developer material consisting of the toner and a carrier of larger granular beads formed with glass, sand or steel bores coated with a material which is removed in the triboelectric series from the toner so that there is a triboelectric charge attraction between the toner and the carrier. The charge causes the toner to adhere to the carrier making it easily handled in a developer system which brings the developer into contact with the previously exposed xerographic surface. Because the charge pattern on the xerographic surface has a greater attraction for the toner than the triboelectric charge the toner has with the carrier the toner is then attracted electrostatically to the image on the surface forming a visible toner image thereon.

United States Patent 3,540,806 Patented Nov. 17, 1970 Excellent results have been obtained in developing systems for a xerographic apparatus using cascade development with toner-carrier (developer) mixtures. This is especially true in continuous xerographic machines used for line copying of documents such as letters or other materials containing dark data on a light background.

Due to electric field conditions in the regions of the electrostatic images, however, large solid areas do not develop uniformly. Xerographic development of such areas delineate only their outline developing only in the areas where there is a differential in charge on the xerographic surface. In these cases the centers of these areas of high charge, being large solid areas of dark input, are not being developed or filled with the toner. Methods have been disclosed for overcoming the difficulties encountered in developing solid area and continuous-tone images with a cascade development system.

L. E. Walkup Pat. No. 2,598,732 discloses a method and apparatus for overcoming these difficulties in stationary flat plate xerographic equipment by exposing the xerographic plate to a screen pattern in addition to the image pattern to be recorded, thereby breaking up the image into a half-tone pattern. However, Walkup does not disclose means which are suitable for application to automatic machines where the surface carrying the electrostatic image is moving such as in the case with a rotating xerographic drum or endless belt photoreceptor.

C. F. Carlson Pat. No. 3,120,790 discloses apparatus for presenting a half-tone pattern to a xerographic member by directly illuminating a rotating xerographic drum through a semi-transparent plate or screen containing opaque lines on a transparent background with the lines running parallel to the rotation of the xerographic member. This screen is positioned to extend over the entire exposure slit of the xerographic apparatus so that various portions of the image are always blocked by the opaque lines of the screen. The screen may be placed in a secondary location forward or aft of the exposure of the object to be copied by the machine. In any case the screen employed must include transparent and opaque lines running parallel to the axis of rotation of the xerographic member, and is continually illuminating throughout the entire exposure step of the xerographic process.

Another method of achieving solid area and continuoustone renditions in xerographic machines is described in the copending application of John M. Faw entitled Xerographic Exposure Apparatus Ser. No. 645,193 filed on June 12,1967.

The apparatus used in the Faw application places an opaque sheet with lines thereon at the object plane of the optical system of the xerographic machine. The lines are parallel to the rotation of the xerographic photoreceptor. The document or object is continually moved at the exposure station of the xerographic apparatus while the xerographic member rotates through its imaging stations optically viewing both the lined opaque screen and the object to be imaged at the object plane of the optical system. The screen extends across the entire width of the document and covers a portion of the object exposure slit being projected to the xerographic member at the exposure station of the xerographic apparatus. The screen has light and dark lines parallel to the direction of motion of the document and the xerographic member and therefore breaks up the charge pattern on the xerographic surface concurrently with the imaging of the document. The application describes the use of apparatus for providing a line screen pattern with a xerographic system in which the document remains stationary while the xerographic member moves by moving the line screen with the optical scanning elements traversing the document. The screen employed must have lines thereon that are parallel to the motion of the photoreceptor.

It is an object of this invention to improve the methods and apparatus for making copies of large solid areas and continuous-tone input on automatic xerographic machines.

It is another object of this invention to permit a greater variety of patterns to be placed on continuously moving photoreceptors in order to half-tone images.

Still another object of this invention is to improve means for shadowing a light source onto a continuously moving xerographic photoreceptor to improve rendition of solid area and continuous-tone images.

These and other objects of this invention are achieved by preiodically exposing a moving xerographic member to a light source through a means to generate a sequence of closely spaced continuing lines or dots or a combination pattern which is placed on the xerographic photoreceptor creating a periodically varying charge pattern. Development of the image on the photoreceptor renders solid area and continuous-tone images of an object previously or subsequently imaged onto the photoreceptor.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of half-toning apparatus embodied in a xerographic mechanism;

FIG. 2 is a schematic representation of another embodiment of half-toning apparatus in another xerographic embodiment;

FIG. 3 is a portion of a schematic representation of the further adaptation of half-toning apparatus;

FIG. 4 is a plan view, partly broken away, of the ap paratus schematically shown in FIG. 3;

FIG. 5 shows a schematic electrical diagram of a circuit for achieving the required illumination of the apparatus in FIGS. 1 and 2.

There is shown in the figures a xerographic copying apparatus comprising a xerographic member or photoreceptor formed in the shape of a drum 20 in FIG. 1 or a belt 22 in FIG. 2. The drum of FIG. 1 is mounted on a shaft 24 suitably journaled to rotate in the direction indicated by the arrow to cause the drum surface to sequentially pass a plurality of xerographic processing stations. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the xerographic member may be described functionally as follows:

A charging station A at which a uniform electrostatic charge is deposited on the photoconductive layer of the drum 20;

An exposure station located preferably as shown by reference character B at which a light or radiation pattern of copy is exposed onto the xerographic photoreceptive surface to dissipate the drum charge in the exposed portions thereof forming a latent electrostatic image of the pattern of the object 26 to be copied;

Adjacent the exposure station is an exposure mechanism generally designated 30 to prepared charge pattern on the drum for development of solid area and continuous-tone images;

A developing station C whereat the latent electrostatic image is developed by cascading an electrostatic developer over the drum forming a toner image corresponding to the latent electrostatic image on the drum;

An image transfer station D where the toner image is electrostatically transferred from the drum surface to a transfer material; and a cleaning station E where the drum is cleaned of residual toner and is discharged in order to prepare the drum surface for the next cycle.

The transfer material then passes through a fusing station F where heat is applied to the material in a suflicient quantity to melt the electrostatically deposited toner image thereon forming a bond between the toner and the transfer material to which it is adhered.

In FIG. 1 the xerographic drum 20 is rotated about its shaft 24 by suitable drive means (not shown). While it is rotating, a scanning system operates at the exposure station through lamps 32 and lens 34 presenting an image of the object 26 at the xerographic drum 20 under the exposure slit 36. For a more detailed description of scanning apparatus such as the type that is schematically shown see Rutkus Pat. No. 3,062,095 issued Nov. 6, 1962. Positioned near the exposure slit 36 is the exposure mechanism for providing the charged surface of drum 20 with capabilities of development of solid area and continuous-tone image formations on the drum. The exposure mechanism shown may be positioned along the drum surface anywhere between the charging station A and the developer station C.

The exposure mechanism 30 is comprised of a light baflle 38 having a narrow slit 40 therein such slit being approximately .04 inch across and extending the entire length of the drum 20 transverse to its rotation. Enclosed within the light baffle 38 is a lamp of such construction as to extend transverse across a substantial portion of the drum surface 20. The lamp may be of a fluorescent, incandescent, xenon, mercury or other gaseous filled type. The lamp is electrically connected through connectors 42 to a suitable electric circuit indicated by box 44. A circuit that would operate on the embodiment shown in FIG. 1 as well as that of FIG. 2 is schematically represented in FIG. 5.

An example of an apparatus of the type shown for creating a half-tone line pattern on the moving xerographic surface of drum 20 could operate as follows:

The drum rotates with a surface speed of approximately two inches per second. The slit 40 in the light baffle 38 is .04 inch across and extends transversely across the entire length of the drum. The slit 40 is positioned approximately .04 inch from the moving surface of the drum 20 without interfering with the motion thereof. This spacing prevents diffusion and spreading of the light rays from the slit 40 to the drum surface. The lamp 46, when ignited, will expose the xerographic drum surface to a shadow of the slit 40. The shadow will expose approximately .04 inch on the surface since the slit is placed close to the moving surface. If the lamp is ignited and turned off times per second, there will be placed on the drum 50 light and dark lines transversely oriented to the rotation of the drum,

The orientation of these lines, which in this example are parallel to the axis of the drum, may be shifted by rotating the light baffle and curving it to fit closely across the width of the drum. The slit 40 and the lamp 46 may also be of a curvilinear rather than straight design. This would cause a series of curved light lines around the drum surface as the lamp 46 is ignited. By providing for 50 ignitions per second of the lamp as in the example cited above, a pattern having a separation of 0.4 inch exposes the previously uniformly charged drum thus dissipating the charge in direct relation to the light exposure on the drum leaving lines of relatively high and low charges adjacent each other on the drum surface.

FIG. 2 shows a xerographic apparatus similar to that in FIG. 1. A xerographic belt 22 is rotated through drive roller 48 and around idle rollers 50 and 52. The belt is exposed at station B along a flat portion of the belt by flashing high intensity lamps such as xenon flash tubes at a platen '54 containing an object 56 causing a reflection creating a radiation pattern for exposing the belt 22.

At a portion of the belt between the charging station A and the developing station C is positioned an exposure mechanism to present a finely-divided uniformly-distributed pattern of light and shadow to the belt prior to developing. The mechanism here operates through a light baffle 58 having a finely-divided pattern 60 in the aperture 62 formed in the baffle. A variety of screen patterns may be used in the aperture 62. Transparent areas may comprise rows of small square entirely surrounded by black or opaque areas. A checkerboard pattern of transparent and opaque areas may be used. The transparent area may be continuously covered with circular black or opaque dots. It may be transparent dots or squares or diamonds or other similar design on a black or opaque background. And there may be alternate rulings of transparent and opaque lines. Any of several sizes of alternate rulings or designs may be used in the pattern 60. A coarse pattern having 50 or 60 dots or lines to the linear inch could be useful for some purposes such as the example cited above. Finer patterns having 100 to 400 and even more dots or lines to the inch will give more nearly a continuous tone appearance to finished copy or print. A lamp 46 is ignited through suitable circuitry in box 44 which connects to the lamp through electrical connectors 42. By igniting the lamp 46 for a relatively short period of time in the order of a few microseconds, a shadow of pattern 60 is placed on the belt while it moves at a constant speed through the several xerographic processing stations. If the aperture 62 in the light bafiie is /2 inch long and the belt has a linear velocity at its surface of inches per second there would be twenty ignitions of lamp 46 per second to place a continual shadow of pattern 60 on the belt. It should be noted that either the exposure mechanism of FIG. 1 or FIG. 2 may be used with the opposite xerographic embodiment from that shown in the respective figures. An important aspect is to keep the pattern or the slit to be shadowed on the xerographic member as close to the xerographic member as practicable to prevent undue spreading of light on the xerographic surface.

FIGS. 3 and 4 schematically show a shutter mechanism for use in presenting a uniformly distributed finely divided pattern to a xerographic surface such as the surface shown in FIG. 3. Here a lamp 64 is constantly illuminated through a suitable power source 66 while a shutter mechanism 68 operates to periodically expose the moving xerographic surface 20 to a pattern 70 positioned at an aperture 72 in the shutter mechanism 68.

The lamp 64 may be of the fluorescent, xenon, mercury or other vapor filled type but more likely in this embodiment would be an incandescent tungsten filament lamp. Its power source 66 would keep it illuminated for as long as the machine is operated. The effect of supplying periodic illumination to the sensitive surface 20 is achieved by mechanically operating the shutter mechanism 68. This is accomplished by pulsing a solenoid 74 which is pinned to the shutter 76 by pin 78. When the solenoid is activated the plunger moves in a direction causing the shutter 76 to ride along guide pins 80- through slots 82 in the shutter 76. The slots 82 are so positioned that the shutter moves to expose the pattern 70 in the aperture 72 of the shutter mechanism to the illumination of lamp 64, The operation of the solenoid 7-4 is rapid and the shutter gate 76 quickly returns to its original position as shown in FIG. 4 thus blocking the light from lamp 64 through pattern 70 to the sensitive surface 20.

The mechanism described is merely illustrative of any mechanism that can cyclically expose the sensitive surface 20 to a light source through a slit such as slit 40 or a pattern such as pattern 70.

FIG. 5 shows one embodiment that may be used to cause a periodic flashing of a lamp such as the lamps 46 in the previous figures. In the figure there is shown a lamp enclosed in an envelope 90 containing a material capable of transmitting desired wave lengths of radia tion at a high intensity to discharge the photoconductive material 82 forming the xerographic plates. Lead-in wires 91 are embedded in the envelope, each lead-in wire bearing an electrode maintained in a spaced apart relationship.

The anode electrode 92 is connected by an electrical conductor 94 to a positive terminal of a capacitor 96 that may be charged through a charging resistance 97 from an energy source such as battery 98 when switch 99 is closed. The other or cathode electrode 93 is connected to the negative terminal of capacitor 96. In combination with the lamp charging circuit is a trigger circuit 102 including, for example, a radio frequency source 103 and an external winding 104.

The trigger circuit provides for an alternative manner to ignite the lamp. For example, capacitor 96 may be charged, as previously described, to a voltage below the breakdown voltage of the particular conditions of the fiash lamp 101. The lamp may then be triggered by means of a trigger circuit 102 transmitting an impulse from radio frequency source 103 to the external winding 104 to cause a partial ionization of the gaseous medium within the envelope making the medium conductive enough to permit the voltage stored in the capacitor 96 to become discharged through the gaseous medium, from the anode 92 to the cathode 93 thereby producing a high intensity radiation in the wave length regions required.

Many other trigger devices are suitable for use herein. For example a charged silver strip painted on part of the envelope or charged metallic reflector can also serve as an external trigger electrode causing the lamp to fire. The lamp itself may be filled with xenon, mercury vapor, or any other gases which would present the desired radiometric results for discharging the xerographic plate. There are presently available xenon lamps having high fre quency, low energy flashes that have a lamp life of 10 flashes to 50% or initial output providing 600 pulses per second at .008 joule per pulse. This and other lamps may be used with the apparatus schematically described in this application for achieving the results stated herein.

While the instant invention has been described with reference to the structures disclosed herein, it is not con fined to the specific details set forth, and this application is intended to cover such modifications or changes as may become within the scope of the following claims.

What is claimed is:

1. In an automated electrostatic reproducing apparatus wherein a photoconductive surface is continually moved past processing stations which provide it with a substan tially uniform electrostatic charge and exposure to a light image of the subject to be reproduced, the method of improving large area reproduction and continuous tone rendition comprising continuously and intermittently exposing the moving surface to a finely divided patterned light source at a rate so as to produce an integrated, uninterrupted pattern of light on the continually moving surface.

2. The method of claim 1 wherein the intermittent exposure of the surface to the light source is through a narrow slit and is repeated at such a frequency as to provide an uninterrupted and uniform series of light and dark lines on the moving surface.

3. The apparatus of claim 6 wherein said means to intermittently prevent the ligh rays from said light source from striking the surface includes a shutter, timing means connected to said shutter to operate said shutter.

4. The apparatus of claim 6 said exposure device further including a light baffle surrounding said light source said light bafiie maintained relatively close to the surface.

5. The apparatus of claim 6 said light bafl'le having an aperture therein including a semi-transparent screen embodying a pattern said light bafiie positioned between said light source and the moving surface.

6. An exposure device for use in an electrostatic reproducing apparatus for breaking up the electrostatic charge on a moving photoconductive surface into a finely divided, uniformly distributed pattern comprising:

a light source having light rays capable of striking the moving surface for exposing the same,

a pattern positioned between the light source and the surface and through which light rays project for exposing the surface, and

' 7 r 1 8 means for intermittently preventing the light rays from 3,120,790 2/1964 Carlson et a1 3553 said light source from striking the surface and at a 3,248,216 4/ 1966 Weigl 355-3 rate relative to the speed of movement of the surface to produce an integrated image of the pattern as a NORTON ANSHER, Primary Examlnel composite of separate exposures of the surface. 5 D. CLEMENT, Assistant Examiner References Cited U S C1. X R UNITED STATES PATENTS 355 17, 11, 8 3,013,890 12/1961 Bixby 355-17 

